1. Field of the Invention
The present invention relates to a polarization splitting device, a method of manufacturing a polarization beam splitter, an optical scanning device, and an image forming apparatus. More particularly, the invention relates to a polarization splitting device that splits lights that are polarized in different directions, a method of manufacturing a polarization beam splitter for use in the polarization splitting device, an optical scanning device including the polarization splitting device, and an image forming apparatus including the optical scanning device.
2. Description of the Related Art
Image forming apparatuses that record electrophotographic images by using laser beams have been widely used. Such an image forming apparatus typically includes an optical scanning device that scans a surface of a photosensitive drum with laser light in an axial direction of the drum by using a deflector (e.g., a polygon mirror) while rotating the drum to thereby form a latent image on the surface of the drum.
In recent years, tandem-type image forming apparatuses that include a plurality of (typically four) photosensitive drums have come into widespread. The tandem-type image forming apparatuses permit high-speed color image formation.
A tandem-type image forming apparatus is likely to be upsized with the increase in the number of the drums. How to make the units of the image forming apparatus, such as an optical scanning device, compact has been an issue. One approach is to superimpose a plurality of optical paths of scanning light beams traveling from a deflector to a corresponding one of photosensitive drums.
For example, Japanese Patent Application Laid-open No. 2008-70599 discloses a conventional optical scanning device. This optical scanning device includes a light source device that emits a plurality of light beams of different polarization directions, a single optical deflecting unit that performs deflection scanning in a main-scanning direction with the light beams emitted from the light source device, a scanning-image forming unit that causes each of the light beams to be converged on a corresponding one of a plurality of surfaces to be scanned, and an optical element that is an optical-path branching unit that is positioned in an optical path from the optical deflecting unit to a corresponding one of the scanned surfaces and that branches an optical path of each of the light beams depending on the polarization direction of the light beam. A projections-and-pits structure whose pitch is equivalent to the wavelength of the light beam or smaller is formed on the optical element.
Japanese Patent Application Laid-open No. 2008-116965 discloses another conventional optical scanning device. This optical scanning device includes a microstructure grating in which grating pitch decreases with increasing incident angle and grating arrangement direction is horizontally or perpendicularly oriented to polarization direction. The microstructure grating is arranged on the surface of an optical element so that Fresnel (surface) reflection from lens surface. This structure is advantageous in reducing a flare spot or ghost light.
Japanese Patent Application Laid-open No. S60-32019 discloses a conventional recording apparatus. This recording apparatus includes two laser light sources that emit two laser beams linearly polarized to be perpendicular to each other and have luminous intensities modulated based on signals to be recorded, a polarized-light combining unit that combines the two laser light beams emitted from the laser light sources together, a deflecting unit that deflects the combined laser light in a main-scanning direction, and a polarized-light splitting unit that splits the combined laser light deflected by the deflecting unit to light beams into different spots on a scanning-recording surface.
Japanese Patent Application Laid-open No. H7-144434 discloses another conventional optical scanning device. This optical scanning device includes a single laser light source that emits laser light, an information control unit that imparts different information pieces to two polarized light beams emitted from the light source, a polarization control unit that controls a degree of polarization based on data from the information control unit, a scanning unit that scans a predetermined illumination surface with the light beam polarized by the polarization control unit, a splitting unit that splits the scanned light beam into two light beams depending on a state of polarization, and an angle-of-rotation control unit that controls an angle of rotation of the laser light from the scanning unit depending on an incident angle on the splitting unit.
However, the optical scanning device disclosed in Japanese Patent Application Laid-open No. 2008-70599 is disadvantageous in that when the incident angle of light incident on the optical-path branching unit is large, performance of the optical-path branching unit in polarization splitting can degrade.
The optical scanning device disclosed in Japanese Patent Application Laid-open No. 2008-116965 is disadvantageous in that it is difficult to achieve sufficient performance in splitting polarized light by using the optical element that includes the microstructure grating.
The recording apparatus disclosed in Japanese Patent Application Laid-open No. S60-32019 is disadvantageous in that an electric-field vector of P-polarized light that is expected to transmit through the polarization splitting unit is not parallel to a transmission axis of a beam-splitting surface in some cases, where a portion of incident light is undesirably reflected from the polarization splitting unit.
The optical scanning device disclosed in Japanese Patent Application Laid-open No. H7-144434 is disadvantageous in that employment of a magnetic optical element as the angle-of-rotation control unit leads to higher cost. Moreover, the act of performing the control of the angle of rotation entails increases power consumption and it leads to more heat generation. It is difficult to control the optical scanning device so as to provide its performance stably because the angle of rotation is likely to fluctuate with environmental conditions such as a temperature.